proportional control
Separate proportional control is also called "differential control". The change of output is proportional to the deviation of input controller, and the greater the deviation, the greater the output. In practical application, the size of the proportional band should be determined according to the specific situation. The proportional band is too small, the control function is too weak, which is not conducive to the system to overcome the disturbance, the residual is too large, the control quality is poor, and there is no control function; If the proportional band is too large and the control effect is too strong, the stability of the system will become worse and oscillation will be caused.
For the controlled object with sensitive response and strong amplification ability, in order to improve the stability of the system, the proportional band should be slightly smaller; For the controlled object with slow response and weak amplification ability, the proportional band can be larger to improve the sensitivity of the whole system and reduce the residual accordingly.
Simple proportional control is suitable for occasions with small disturbance, small lag, small load change and low requirements, and a certain margin is left. Proportional control law is widely used in industrial production.
proportional plus integral control
Proportional control law is one of the most basic and widely used basic control laws, and its greatest advantage is timely and fast control. As long as there is deviation, the controller will control it immediately. However, the disadvantage that the residual error cannot be finally eliminated limits its independent use. The way to overcome the residual error is to add integral control on the basis of proportional control.
The output of the integral controller is proportional to the integration of the input deviation with time. The "integral" here means "accumulation". The output of the integral controller is not only related to the magnitude of the input deviation, but also related to the time when the deviation exists. As long as the deviation exists, the output will continue to accumulate (the output value will become larger or smaller), and the accumulation will not stop until the deviation is zero. Therefore, the integral control can eliminate the residual error. Integral control law is also called indifference control law.
The magnitude of integration time indicates the intensity of integration control. The smaller the integration time, the stronger the control effect; On the contrary, the weaker the control effect.
Although integral control can eliminate residual error, it has the disadvantage of untimely control. Because the accumulation of integral output is gradual, its control effect always lags behind the variation of deviation, so it is difficult to overcome the influence of interference in time and effectively, and the control system is difficult to be stable. Therefore, in practical application, integral control is not used alone, but combined with proportional control to form proportional integral control. In this way, we can learn from each other's strengths, which not only has the function of fast and timely proportional control, but also has the ability of integral control to eliminate residual. Therefore, proportional integral control can realize ideal process control.
Proportional integral controller is one of the most widely used controllers at present, which is mainly used in liquid level, pressure and flow control systems in industrial production. Because integral can eliminate residual error, make up for the defects of pure proportional control and obtain better control quality. However, the introduction of integration will make the stability of the system worse. The control system with large inertia lag should be avoided as far as possible.
proportional plus derivative contro
Proportional integral control is not ideal for the controlled object with time delay. The so-called "time delay" means that when the controlled object is disturbed, the controlled variable does not change immediately, but there is a time delay, such as capacity lag, when the proportional-integral control is slow and untimely. To this end, people imagine: can we make corresponding control actions according to the changing trend of deviation? Just like an experienced operator, he can change the valve opening according to the deviation (proportional effect), predict what will happen according to the speed of deviation change, control the excess in advance, and achieve "nip in the bud". This is the differential control law with "advanced" control function. The output of the differential controller depends on the speed at which the input deviation changes.
The differential output is only related to the change speed of deviation, and has nothing to do with the size and existence of deviation. If the deviation is a fixed value, no matter how big it is, as long as it remains unchanged, the change of output must be zero, and the controller has no control function. The longer the differential time, the longer the differential output duration, so the stronger the differential effect; On the contrary, it is weaker. When the differential time is 0, there is no differential control. Similarly, the choice of differential time also needs to be determined according to the actual situation.
The characteristics of differential control are: quick action and advanced adjustment function, which can effectively improve the control quality of the controlled object with large time delay; But it can't eliminate the residual error, especially for constant deviation input, and it has no control effect at all. Therefore, the differential control law cannot be used alone.
The combination of proportional and differential action is faster than simple proportional action. Especially for the object with large capacity lag, it can reduce the amplitude of dynamic deviation, save control time and significantly improve control quality.
Proportional integral differential (PID) control
The most ideal control is proportional-integral-differential control law. It combines the advantages of the three: it has timely and rapid proportional action, the ability to eliminate residual errors through integral action and the advanced control function through differential action.
When the deviation jumps out of the present, the differential immediate action suppresses this deviation jump; Proportion also plays a role in eliminating deviation and reducing deviation range. Because proportional action is a lasting and main control law, the system can be relatively stable. The integral action gradually overcomes the residual difference. As long as the control parameters of the three functions are properly selected, the advantages of the three control laws can be fully exerted and the ideal control effect can be obtained.
Edit the debugging method of PID controller in this paragraph.
Adjustment of proportional coefficient
The adjustment range of the proportional coefficient p is generally 0. 1- 100.
If the gain value is 0. 1, the output of PID regulator becomes one tenth of the deviation value. If the gain value is 100, the output of PID regulator becomes a deviation value of one hundred times.
It can be seen that the larger the numerical value, the greater the gain effect produced by the proportion. In the initial adjustment, choose a smaller one, and then slowly increase it until the fluctuation of the system is small enough, and then adjust the integral or differential coefficient. Excessive p value will lead to system instability and continuous oscillation; Too small a value of p will slow down the system. The appropriate value should make the system sensitive enough but not too sensitive, and the delay of a certain time depends on the integration time.
Adjustment of integral coefficient
The integration time constant is defined as the time when the deviation causes the output to increase. If the integration time is set to 1 sec, the time required for the output to change 100% is 1 sec. During the initial adjustment, the integration time should be set longer, and then gradually reduced until the system is stable.
Adjustment of differential coefficient
Differential value is the rate of change of deviation value. For example, if the input deviation value changes linearly, a constant adjustment amount is superimposed on the output side of the regulator. Most control systems do not need to adjust the differential time. Because only systems with time delay need to attach this parameter. If this parameter is increased, the control of the system will be affected. If the ideal control requirements cannot be met by adjusting the proportional and integral parameters, the differential time can be adjusted. The coefficient is set to be small during the initial adjustment, and then gradually increased until the system is stable.
Parameter tuning of PID controller is the core content of control system design. According to the characteristics of the controlled process, the proportional coefficient, integral time and differential time of PID controller are determined. There are many methods for tuning PID controller parameters, which can be summarized into two categories: one is theoretical calculation tuning method. It mainly determines the controller parameters through theoretical calculation according to the mathematical model of the system. The calculated data obtained by this method cannot be used directly, and must be adjusted and corrected through engineering practice. The second is the engineering setting method, which mainly relies on engineering experience and is directly carried out in the test of control system. This method is simple and easy to master, and is widely used in engineering practice. The engineering tuning methods of PID controller parameters mainly include critical proportion method, response curve method and attenuation method. The three methods have their own characteristics, and the similarities are all through experiments, and then the parameters of the controller are adjusted according to the engineering experience formula. But no matter which method is adopted, the parameters of the controller need to be finally adjusted and improved in actual operation. At present, the critical proportion method is generally used. The steps of tuning PID controller parameters by this method are as follows: (1) First, preselect a sampling period short enough to make the system work; (2) Only add the proportional control link until the step response of the system to the input appears critical oscillation, and write down the proportional magnification and critical oscillation period at this time; (3) Under a certain degree of control, the parameters of PID controller are calculated by formula.
In actual debugging, only an empirical value can be roughly set first, and then modified according to the adjustment effect.
For temperature system: P (%) 20-60, I (min) 3- 10, D (min) 0.5-3.
For mobile system: P (%) 40- 100, I (min) 0. 1- 1.
For pressure system: P (%) 30-70, I (min) 0.4-3.
Liquid level system: P (%) 20-80, I (min) 1-5.
Parameter adjustment finds the best order from small to large.
Proportion first, then integration, then differentiation.
The curve oscillates frequently, so the proportional reel should be enlarged.
The curve floats around Dawan, and the proportional band turns into a small pull.
The recovery of curve deviation is slow and the integration time is reduced.
The curve has a long fluctuation period and a long integration time.
The oscillation frequency of the curve is fast, and the differential is reduced first.
The dynamic difference is large and the fluctuation is slow. The difference time should be longer.
There are two waves in the ideal curve, which are 4 1 higher before and lower after.
At first glance, the quality of adjustment will not be low.